Various features of the biology of the rust fungi and of the epidemiology of
the plant diseases they cause illustrate the important role of rainfall in
their life history. Based on this insight we have characterized the ice
nucleation activity (INA) of the aerially disseminated spores (urediospores)
of this group of fungi. Urediospores of this obligate plant parasite were
collected from natural infections of 7 species of weeds in France, from
coffee in Brazil and from field and greenhouse-grown wheat in France, the
USA, Turkey and Syria. Immersion freezing was used to determine freezing
onset temperatures and the abundance of ice nuclei in suspensions of washed
spores. Microbiological analyses of spores from France, the USA and Brazil,
and subsequent tests of the ice nucleation activity of the bacteria
associated with spores were deployed to quantify the contribution of
bacteria to the ice nucleation activity of the spores. All samples of spores
were ice nucleation active, having freezing onset temperatures as high as
−4 °C. Spores in most of the samples carried cells of ice
nucleation-active strains of the bacterium <i>Pseudomonas syringae</i> (at rates of less than 1
bacterial cell per 100 urediospores), but bacterial INA accounted for only a
small fraction of the INA observed in spore suspensions. Changes in the INA
of spore suspensions after treatment with lysozyme suggest that the INA of
urediospores involves a polysaccharide. Based on data from the literature,
we have estimated the concentrations of urediospores in air at cloud height
and in rainfall. These quantities are very similar to those reported for
other biological ice nucleators in these same substrates. However, at cloud
level convective activity leads to widely varying concentrations of
particles of surface origin, so that mean concentrations can underestimate
their possible effects on clouds. We propose that spatial and temporal
concentrations of biological ice nucleators active at temperatures
> −10 °C and the specific conditions under which they
can influence cloud glaciation need to be further evaluated so as to
understand how evolutionary processes could have positively selected for
INA.